Generally, a CRT is designed to realize an image by electron-beams emitted from an electron gun and scanning a phosphor screen deposited with red R, green G and blue B phosphors. The CRT typically includes a panel with a phosphor screen at its inner surface, an electron gun for emitting electron beams, a neck portion for receiving the electron gun, a funnel for connecting the panel to the neck portion, and a color selection apparatus mounted on an inner surface of the panel.
The color selection apparatus is disposed facing the phosphor screen to let the electron beams emitted from the electron gun land on the corresponding phosphors. The color selection apparatus includes a shadow mask functioning as an electrode, a mask frame for supporting the shadow mask, and a spring assembly for fixing the mask frame on the panel.
The shadow mask is formed by drawing AK steel or INVAR steel after a plurality of beam-passing apertures are formed on the steel through a photolithography process. Since the shadow mask is very thin when compared with its large area and hundreds of thousands of apertures are formed on such a thin plate, its strength is very low. Therefore, the shadow mask may be easily depressed by outer shock or domed toward the phosphor screen by the thermal expansion caused by electron beams emitted from the electron gun.
When the shadow mask is depressed or domed, the location of the beam-passing apertures is shifted. This may deteriorate the color purity as the shadow mask cannot precisely select the color.
To develop a shadow mask suitable for the trend toward the large-sized and flattened CRT while solving the above described problems, Japanese laid-open patent No. H7-230760 discloses a tensioned mask that is fixed on the frame in a state where tension is applied thereto.
That is, as shown in FIG. 4, tensioned mask 104 provided with a plurality of beam-passing apertures 102 is tensioned in a vertical direction (in a direction of a Y-axis in the drawing) and welded on supporting members 106 that are connected to each other by a pair of elastic members 108.
Supporting members 106 and elastic members 108 define mask frame 110 supporting tensioned mask 104.
Spring assembly 112 for mounting mask frame 110 on an inner surface of the panel is mounted on each sidewall of supporting members 106 and elastic members 108.
Spring assembly 112 includes hook 114 having one end welded on supporting member 106 or elastic member 108 and spring 116 having a first end fixed on hook 114 and a second end coupled on a stud pin (not shown) buried on an inner surface of the panel through coupling hole 116a. Hook 114 includes a high thermal expansion member and a low thermal expansion member that are joined to each other lengthwise.
Tensioned mask 104 and mask frame 110 that are integrally welded goes to a heat-treatment process including a darkening process. The darkening process is for forming a dark layer on tensioned mask 104 by injecting a mixture gas of air and propane into a darkening furnace having a temperature of about 610˜620° C.
However, when mask frame 110 is subjected to a variable heat-treatment process and the process varies less than desired, tensioned mask 104 may undergo elastic deformation due to the loss of its tensile force. The CRT's display quality can then become deteriorated.
To solve this problem, reinforcing member 118 having a thermal expansion coefficient higher than elastic members 108 is welded on a lower portion of elastic members 108. Alternatively, a reinforcing member having a thermal expansion coefficient lower than elastic members 108 is welded on an upper portion of elastic members 108. Reinforcing member 118 is deformed during the heat-treatment process so that tensioned mask 104 is applied with a retraction force in a the curved arrow directions of FIG. 4, thereby suppressing the elastic deformation of tensioned mask 104.
In addition, reinforcing member 118, hook 114, and tensioned mask 104 sequentially go through the welding process in this order by a resistance welding at two or three welding points 120.
Here, the resistance welding is performed by applying a current of 6,000˜7,000 amperes in a short time in a state where members to be welded to each other are overlapped and applied with a predetermined pressure, thereby welding the members to each other by melting the members using a heat resistance generated by a contact resistance between the members.
However, when welding the reinforcing member and the hook to the mask frame, the welding does not occur simultaneously but occurs in a predetermined time sequence.
Accordingly, due to the high heat generated during the welding of the first welding point, the members (reinforcing member, mask frame and etc.) to be welded increase in temperature. As a result, the contact area of the welding electrode is enlarged during the welding of the second and third welding points, thereby forming oxide layer which deteriorates the electric conduction.
Therefore, the welding force at the second and third welding points is lower than that at the first welding point, deteriorating the welding reliability. When a sample frame is used for checking the welding force to improve the welding reliability, the costs are increased.
Furthermore, since the high current of 6,000˜7,000 amperes is used for the resistance welding, the mask frame is magnetized at a magnetic flux density of about 60˜70 gausses after the welding process is finished.
Accordingly, a degaussing process for reducing the magnetic flux density inevitably needs to be performed for the magnetized mask frame.
Therefore, the present invention has been made in an effort to solve the above-described problems.